Sure, there were multi-path intercom systems available. The AT&T/Western Electric 6A intercom was huge (as in size; all 23" rack-mounted relays that got bigger by the number of stations) but it still only had two paths. The first was a common path shared by everyone, but once a call was dialed, the parties were shifted to a second private path. Once the private path was in use, the next intercom call was on the public path. A third call attempt became a party line.
This was also when people actually got up from their desks every once in a while as opposed to today! If the intercom paths were busy, people just got up and went to the person they were trying to call!
AT&T/Western Electric came out with an advanced intercom system that was similar to a small PBX that was called "Dialog". It had many talk paths and functioned much like a PBX with built-in lamp flash rates for the 1A2 sets, etc. It even had clear plastic overlays for the faceplates on the phones to provide feature dialing codes. It was pretty sophisticated for it's time. Maybe that's why it didn't last very long.
Tone Commander also came out with the Multi-link intercom system for 1A2 called the ML-8000. With the proper cards, I seem to recall that it could go up to 8 talk paths. It also functioned much like a PBX. All intercom call paths were private. Ringback and busy tones were provided when making calls (this was an option). Once the equipped path capacity was reached, the next call attempt resulted in a busy signal when going off-hook. These things were VERY popular in the DC area through the 1980's.
Both of these systems provided "real" ringing or signaling (repeated) as opposed to the single buzz with earlier intercom systems.
Melco and Valcom had multi-path systems as well, although not quite as sophisticated.
Transfer keys were more for ringing on CO lines. Say for example an executive was going out to lunch, he could turn on his transfer key (a side car toggle switch) and his private line would then ring at the receptionist.
